In recent years, energy consciousness has dictated much tighter construction of home and commercial dwelling units. As one aspect of this, there are now fewer internal air turnovers for any unit of time. In modern mobile homes, as one example, when doors and windows are closed, air turnovers typically range from 0.15 to 0.6 per hour, averaging about 0.3. When this tight construction has indeed helped to conserve energy, it has spawned a host of new problems. Among these are those caused by internally generated moisture and odors. In this latter category, minute quantities of formaldehyde vapor generated from a multitude of internal sources can be cited as one example. The sources of formaldehyde include tobacco smoking, natural gas cooking, carpeting, permanent press treatment of draperies and upholstering fabrics, and the urea-formaldehyde adhesives used in particleboard decking and in wall paneling. Formaldehyde evolution is usually more of a problem in hot, humid weather. The amounts emitted are very low. The ambient atmosphere in mobile homes currently being manufactured will rarely exceed 1.5 ppm and will typically be about 0.2 to 0.6 ppm.
To cure the problem, it is necessary to control all of the sources of formaldehyde. Unfortunately, elimination of one or two sources does not guarantee that the remaining level of formaldehyde will be reduced in proportion to the original contribution of these sources. Effort must be made along multivariate lines of attack to completely resolve the formaldehyde problem. One part of this effort, as represented by the present invention, has been to better control formaldehyde emission from particleboard bonded by formaldehyde-containing adhesives. Particlebaord decking and cabinetry core products are widely used in mobile homes and are usually regarded as significant contributors to the internal formaldehyde levels. The art has long recognized that formaldehyde emission from composite products, such as particleboard bonded with urea-formaldehyde resins, can be in part controlled by the addition of free urea to the system. Urea acts as a formaldehyde scavenger, both at and subsequent to the time of manufacture of the product. The art is divided into three general ways of accomplishing this end. One way is to add the urea directly to the resin. Here it appears to tie up free formaldehyde that may be present in the resin. In effect, the molar ratio of formaldehyde to urea is decreased. This approach is exemplified in British Pat. No. 1,480,096. It is further discussed in British Pat. No. 2,019,854 and by A. A. Moslemi in Particleboard Volume I, Southern Illinois University Press, Carbondale (1974). This idea is attractive in its simplicity but it has not been particularly successful. Physical properties of the resultant product are severely affected when enough urea is added to gain significant reduction of formaldehyde evolution. In addition to the penalty paid in poorer physical properties, press times are significantly increased.
Another approach developed out of the failure of the one just described. This involved isolation of the urea from the resin in some manner so that it does not interfere with resin curing but is available later to scavenge formaldehyde. This often involves applying a solution of urea to the wood particles, for example as taught in British Pat. No. 2,019,854; or to a portion of the particles as in German Pat. No. 1,653,167; or to an inert carrier material, as in U.S. Pat. No. 3,983,084, in which a mixture of urea and an amylaceous material is added to the binder system. Again, the ideas are excellent in concept but in actuality none of these references cited seem able to reduce formaldehyde evolution to more than about 50% of the untreated level. Much better control has been a sought-after goal. Lehmann, in U.S. Pat. No. 4,397,756, teaches a system which has found some commercial success. He adds a solution of urea protected by a carbohydrate-based material either to the particles or to the resin. While this system appears to be effective, it has the disadvantage of adding additional water to the particles. This higher moisture content can adversely affect press time. In extreme cases, it can also cause delaminations known as "blows" due to the formation of steam pockets during pressing.
A third approach can be mentioned. This involves reformulation of the resins themselves, usually in the direction of lower molar ratios of formaldehyde to urea. In recent years resin manufacturers have successfully reduced the molar ratio of formaldehyde to urea from a former level of about 1.4 to 1.6 to about 1.2 or even slightly lower.
Finally, other materials besides urea have been suggested, as in U.S. Pat. No. 4,186,242, in which ammonium lignosulfonate is used, or in U.S. Pat. No. 2,870,041 in which bisulfites are suggested for odor control in textiles. German DT No. 2847-975 shows the use of urea for formaldehyde absorption in pharmaceutical and cosmetic preparations.
Other approaches to reduction of formaldehyde emission have been developed as post-pressing treatments. Moen, in German DE No. 28 29 021, sprays hot panels out of the press with a 10% aqueous urea solution. The sprayed panels are then hot stacked for a period of time. While this treatment may indeed be effective for its intended purpose, it would be unacceptable to most manufacturers since it would cause severe surface roughening requiring heavy sanding. Neumann, in West German No. 29 29 775, proposes storing particleboard in a warm closed room with a volatile ammonium salt that will thermally decompose and release gaseous ammonia. Alternatively, the particleboard can be hot stacked with a layer of an ammonia generating salt, such as ammonium bicarbonate, between adjacent panels. The ammonia thus liberated by the heat reacts with formaldehyde vapor to form hexamethylenetetramine, a compound which is relatively stable at room temperatures. Westling, in E.P.C. publication No. 0,027,583 shows a very similar process. However, Westling notes that while the ammonium bicarbonate treatment is effective in the short term, it does little or nothing to eliminate longer-term problems with formaldehyde. He proposes to counteract this problem by including urea with the ammonium carbonate or bicarbonate. Westling's process apparently requires hot stacking the panels at 60.degree. C. for two days, a requirement that manufacturers would find prohibitively expensive.
Problems of formaldehyde emission have long been a serious handicap to the wide acceptance of urea-formaldehyde foams as a home insulating material. Kelly, et al., U.S. Pat. No. 3,231,525, note that ammonium bicarbonate has been added to UF resins, prior to mixing with the foaming hardener, to control formaldehyde emissions. They propose, instead, adding a small amount of urea to the hardener solution. Justice, U.S. Pat. No. 3,312,639, adds a small amount of ammonium carbonate or bicarbonate to the resin component. Prior to preparing the foam, additional urea is added to the resin to adjust the mole ratio to between 1.6 and 2.0.
These approaches either stray from practicality in the manufacture of wood products or, like the others, fail to give adequate control of formaldehyde evolution. Even more so, do they fail to solve the problem for a secondary manufacturer who must purchase panels on the open market for remanufacture. To the past time, the secondary manufacturer has had little or no control over such aspects of his ultimate product as formaldehyde evolution, even though he would be likely to bear the brunt of any product liability claims.